Hydrocarbon Tail Groups Research Articles

Supramolecular Self-Assembly of Dipalmitoylphosphatidylcholine and Carbon Nanotubes: A Dissipative Particle Dynamics Simulation Study.

Dissipative particle dynamics simulations were performed to investigate the self-assembly of dipalmitoylphosphatidylcholine (DPPC) as a model lipid membrane on the surface of carbon nanotubes (CNTs). The influence of surface curvature of CNTs on self-assembly was investigated by performing simulations on solutions of DPPC in water in contact with CNTs of different diameters: CNT (10, 10), CNT (14, 14), CNT (20, 20), and CNT (34, 34). DPPC solutions with a wide range of concentrations were chosen to allow for formation of lipid structures of various surface densities, ranging from a submonolayer to a well-organized monolayer and a CNT covered with a lipid monolayer immersed in a planar lipid bilayer. Our results are indicative of a sequence of phase-ordering processes for DPPC on the surface of CNTs. At low surface coverages, the majority of hydrocarbon tail groups of DPPC are in contact with the CNT surface. Increasing the surface coverage leads to the formation of hemimicellar aggregates, and at high surface coverages close to the saturation limit, an organized lipid monolayer self-assembles. An examination of the mechanism of self-assembly reveals a two-step mechanism. The first step involves densification of DPPC on the CNT surface. Here, the lipid molecules do not adopt the order of the target phase (lipid monolayer on the CNT surface). In the second step, when the lipid density on the CNT surface is above a threshold value (close to saturation), the lipid molecules reorient themselves to form an organized monolayer around the tube. Here, the DPPC molecules adopt stretched conformations normal to the surface, the end hydrocarbon groups adsorb on the surface, and the head groups occupy the outermost part of the monolayer. The saturation density and the degree of lipid ordering on the CNT surface depend on the surface curvature. The saturation density increases with increased surface curvature, and better-ordered structures are formed on less curved surfaces.

Antifouling Studies of Unsymmetrical Oligo(ethylene glycol) Spiroalkanedithiol Self-Assembled Monolayers

The antifouling properties of self-assembled monolayers (SAMs) on gold generated from custom-designed bidentate unsymmetrical spiroalkanedithiols containing both oligo(ethylene glycol) and hydrocarbon tailgroups (EG3C7-C7 and EG3C7-C18) were evaluated and compared to SAMs derived from analogous monodentate octadecanethiol (C18SH) and the tri(ethylene glycol)-terminated alkanethiol EG3C7SH. Complementary techniques, including in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements, were employed to assess the protein resistance of the SAMs using proteins having a wide range of sizes, structures, and properties: protamine, lysozyme, bovine serum albumin (BSA), and fibrinogen. The studies found that SAMs generated from the bidentate adsorbates EG3C7-C7 and EG3C7-C18, which contain a 1:1 mixture of OEG and hydrocarbon tailgroups, exhibited a diminished capacity to resist protein adsorption compared to the EG3C7SH SAMs, which possess only OEG tailgroups. The data highlight the critical role of hydration of the OEG matrix for generating antifouling OEG-based surface coatings.

Supramolecular gels from alkylated benzimidazolone derivatives

In this work, seven alkylated benzimidazolone (BZI) derivatives were synthesized with different size and shaped hydrocarbon tail-groups connected to the primary H-bonding BZI head-group via either an amide or urea secondary H-bonding linker group. The gelation behavior of these compounds was screened in up to 53 organic liquids with different properties and examined using Hansen solubility parameters (HSPs) in order to identify successful gelators and structural attributes responsible for gelation. Successful gelators possessed an amide linker and a branched hydrocarbon tail-group. The size, shape (degree of branching) and hydrophobicity of the tail-group significantly influenced the gelator ability, rheological properties and network morphology. Spectroscopic, powder x-ray diffraction, and molecular modelling studies were also carried out to provide insight into the self-assembly driving forces and supramolecular structure of gels with low polarity liquids. Finally, cytotoxicity studies suggested that the alkylamido BZI gelators biocompatible and could be used for biomedical applications and may even have some therapeutic properties of their own.

Antifouling Coatings Generated from Unsymmetrical Partially Fluorinated Spiroalkanedithiols

Biofouling negatively impacts modern society on a daily basis, especially with regard to the important industries of medicine, oil, and shipping. This manuscript describes the preparation and study of model antifouling coatings generated from the adsorption of unsymmetrical partially fluorinated spiroalkanedithiols on gold. The antifouling properties of the self-assembled monolayers (SAMs) derived from the spiroalkanedithiols were compared to SAMs derived from analogous monodentate partially fluorinated and nonfluorinated alkanethiols. The antifouling properties were evaluated using in situ surface plasmon resonance spectroscopy (SPR), ex situ electrochemical quartz crystal microbalance (QCM) measurements, and ex situ ellipsometric thickness measurements. The resistance to nonspecific protein adsorption of the SAMs was evaluated with proteins having a wide range of properties and applications including protamine, lysozyme, bovine serum albumin, and fibrinogen. The results from the SPR and the QCM measurements demonstrated that in most cases, the SAM coatings derived from the partially fluorinated spiroalkanedithiols having mixed hydrocarbon and fluorocarbon tail groups exhibited better antifouling performance when compared to the SAMs derived from their single-component monodentate counterparts. The studies also revealed that while the SPR and the QCM measurements in most cases were able to distinguish the adsorption trends for the SAMs and proteins examined, the ellipsometric thickness measurements were markedly less discriminating. On the whole, these studies validate the use of unsymmetrical partially fluorinated spiroalkanedithiols for generating effective antifouling coatings on metal substrates.

The electrochemical phase behaviour of chemically asymmetric lipid bilayers supported at Au(111) electrodes

Chemically asymmetric phospholipid bilayers supported on Au(111) surfaces have been studied with differential capacitance and chronocoulometry measurements. The bilayers were prepared using Langmuir-Blodgett deposition, followed by Langmuir-Schaeffer deposition using a different molecule. Combinations of three phospholipids with common hydrocarbon tail groups were compared: dimyristoyl phosphatidyl choline (DMPC), dimyristoyl phosphatidyl ethanolamine (DMPE) and dimyristoyl phosphatidyl serine (DMPS). DMPC and DMPE are zwitterionic and DMPS is anionic in the electrolyte used. The asymmetric bilayers containing DMPS gave distinct electrochemical responses depending on whether the DMPS was deposited first, onto Au, or second, onto another lipid, indicating that asymmetry was maintained over the timescale of the experiment. The results for DMPE and DMPC combinations suggest similar headgroup conformations in the two bilayers. The relationship between the location of the charged molecule and the electrochemical properties is complex but in all cases, DMPS tends to raise bilayer capacitance and DMPE tends to lower capacitance. These observations can be explained by the relatively higher solvation of the charged DMPS molecules and tight packing of DMPE molecules, which leads to exclusion of solvent from the bilayer.

Synthesis and characterization of asymmetrical gemini surfactants.

The effect of variation in the length of surfactant hydrocarbon tail groups was tested in a series of dissymmetric gemini surfactants (N1-alkyl N1,N1,N3,N3-tetramethyl-N3-(6-pyren-6yl)-hexyl)propane-1,3-diammonium dibromide designated as CmC3CnBr, with m = hexyl pyrene, and n = 8, 12, 14, 16, and 18. The aggregation properties of these surfactants have been investigated by means of 1H NMR, fluorescence spectroscopy, surface tension and electrical conductivity measurements. The critical micelle concentration (CMC) was determined using surface tension and confirmed using the specific conductance method. Krafft temperatures and the degree of micelle ionization (α) were obtained from specific conductance measurements. With an increase of the dissymmetry (m/n) ratio, the CMC decreased linearly and an increase in the Krafft temperatures was observed for all of the gemini surfactants. α values for the dissymmetric GS were higher than those of the m-3-m counterparts, which may be attributed to enhanced micelle-micelle interactions that arise from increased hydrophobicity of the hydrocarbon chains. The introduction of the bulky pyrenyl tail group resulted in much lower CMC values compared to their symmetrical counterparts affecting the packing of these surfactants at the air/water interface, which resulted in high-ordered structures (lamellar and inverted micelles). This in turn affected the thermodynamic parameters of the micellization.

Aromatic Surfactant as Aggregating Agent for Aptamer-Gold Nanoparticle-Based Detection of Plasmodium Lactate Dehydrogenase.

A novel ssDNA aptamer (P38), with a 40 mer random region flanked by primer-binding sites on both sides, targeting Plasmodium falciparum lactate dehydrogenase (PfLDH) has been developed through systematic evolution of ligands by exponential enrichment (SELEX), including counter SELEX against human lactate dehydrogenase A and B (hLDH A and B). The 2D structure of P38 shows the presence of three stem loops with a δG of -9.18kcal/mol. EMSA studies on P38 complexes with the increasing concentration of PfLDH revealed a dissociation constant of 0.35µM. P38 has been exploited for the quantitative detection of PfLDH using cationic surfactant-mediated aggregation of gold nanoparticles (16-nm diameter) as an optical probe. Among the three different cationic surfactants, characterized by different hydrocarbon tail groups, benzalkonium chloride (BCK) was found to be most efficient with a limit of detection of 281±11pM. This BCK-based approach with the novel highly selective aptamer provides simple and sensitive detection of PfLDH in the clinically relevant range.

Nonequilibrium Molecular Dynamics Simulations of Organic Friction Modifiers Adsorbed on Iron Oxide Surfaces.

For the successful development and application of lubricants, a full understanding of the nanoscale behavior of complex tribological systems is required, but this is difficult to obtain experimentally. In this study, we use nonequilibrium molecular dynamics (NEMD) simulations to examine the atomistic structure and friction properties of commercially relevant organic friction modifier (OFM) monolayers adsorbed on iron oxide surfaces and lubricated by a thin, separating layer of hexadecane. Specifically, acid, amide, and glyceride OFMs, with saturated and Z-unsaturated hydrocarbon tail groups, are simulated at various surface coverages and sliding velocities. At low and medium coverage, the OFMs form liquidlike and amorphous monolayers, respectively, which are significantly interdigitated with the hexadecane lubricant, resulting in relatively high friction coefficients. At high coverage, solidlike monolayers are formed for all of the OFMs, which, during sliding, results in slip planes between well-defined OFM and hexadecane layers, yielding a marked reduction in the friction coefficient. When present at equal surface coverage, OFMs with saturated and Z-unsaturated tail groups are found to yield similar structure and friction behavior. OFMs with glyceride head groups yield significantly lower friction coefficients than amide and particularly carboxylic acid head groups. For all of the OFMs and coverages simulated, the friction coefficient is found to increase linearly with the logarithm of sliding velocity; however, the gradient of this increase depends on the coverage. The structure and friction details obtained from these simulations agree well with experimental results and also shed light on the relative tribological performance of these OFMs through nanoscale structural variations. This has important implications in terms of the applicability of NEMD to aid the development of new formulations to control friction.

The design of shape-tunable hairy vesicles

Via the use of a mesoscopic simulation technique called dissipative particle dynamics, we design sterically stable biocompatible vehicles through the self-assembly of a binary mixture composed of amphiphilic molecular species, such as PEGylated lipids, and phospholipids. We examine the factors controlling the shape of the hairy vesicle, and report the shape to change with molecular stiffness, and dissimilarity in the hydrocarbon tail groups, along with the relative concentration of the species, and the functional group length. We also draw correspondence with experimental studies on the shape transformations of the hairy vesicles through phase diagrams of the reduced volume, the ratio of the minimum and maximum radii, and the interfacial line tension, as a function of the concentration of the hairy lipids and the hydrocarbon tail molecular chain stiffness. Results from our investigations can be used for the design and prediction of novel hybrid soft materials for applications in the encapsulation and delivery of therapeutic agents.

Solubilization and Interactions of Thiram with Surfactants: Micellar Effects on Adsorption Characteristics of Thiram onto Activated Carbon

The solubilization of thiram was studied in micellar solutions of two surfactants possessing the same hydrocarbon tail but different hydrophilic head groups, namely sodium dodecyl sulfate and dodecyltrimethylammonium bromide. Conductivity, spectrophotometry, and surface tension measurements have been used in quantitative analysis of the interaction between thiram and surfactants. In this work the adsorption of thiram onto selected two different activated carbons has been also studied in the absence and presence of micelles. The interaction of thiram in different micellar solutions provided information about the characteristics of adsorption properties of poorly soluble pesticide thiram onto activated carbon in the presence of micelles.

Influence of Organic Amino and Thiol Ligands on the Geometric and Electronic Surface Properties of Colloidally Prepared Platinum Nanoparticles

The influence of organic ligands on the geometric and electronic surface properties of colloidally prepared Pt nanoparticles (Pt NPs) was investigated by means of diffuse reflectance infrared spectroscopy (DRIFTS) and using CO as probe molecule. Dodecylamine (DDA) and dodecylthiol (DDT) were used as surface functionalizing ligands that exhibit the same hydrocarbon tail but different functional groups. While for DDA an electronic donor effect as well as a geometric effect was found, the effect of DDT was mainly geometric in nature. For high DDA and DDT coverages, on-top sites were blocked for CO adsorption to a higher extent than bridge and threefold hollow sites. Whereas DDA is only weakly adsorbed and displaced by strongly binding adsorbates (e.g., CO), DDT is strongly attached to the particle surface forming an evenly distributed capping layer. With decreasing thiol coverage, blocking of bridge and threefold hollow sites became more pronounced than on-top site blocking. The influence of both ligands on the selectivity of the hydrogenation of crotonaldehyde catalyzed by Pt NPs was investigated. For amino-functionalized NPs the catalytic properties did not differ from that of “unprotected” Pt NPs. In contrast, an increased selectivity can be found for thiol-functionalized particles, which, according to the IR-spectroscopic investigations, was attributed to a geometric modification of the surface by the ligand.

Effect of Electric Field on Structure and Dynamics of Bilayers Formed From Anionic Phospholipids

The effect of molecular structure on ensemble structure and dynamics of phospholipid bilayers has been investigated. Bilayers of dimyristoyl phosphatidylserine (DMPS) supported on Au(111) surfaces were prepared by Langmuir-Blodgett and Langmuir-Schaeffer deposition and studied with a combination of electrochemical measurements and in situ Polarisation Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). DMPS bilayers have relatively large capacitance when compared with those formed from similar molecules and this is attributed to a high solvent content within the bilayer, resulting from the need for solvation of the negatively charged lipid headgroups. Infrared spectra show that the ensemble of molecules is in a gel state, with extended and ordered hydrocarbon chains, similarly to bilayers of dimyristoyl phosphatidylethanolamine (DMPE) molecules, which are of similar shape. The infrared spectra also show that, in contrast to DMPE, the headgroups of DMPS are very strongly hydrated and have higher mobility. This higher mobility allows the re-orientation of the molecules under the influence of an applied electric field: re-orientation both of headgroups and hydrocarbon tail groups is observed. Thus the shape and charge of the molecules in an ensemble have a strong influence on both their structure and dynamics in the presence of an externally applied electric field.

Immersion Depth of Surfactants at the Free Water Surface: A Computer Simulation and ITIM Analysis Study

The adsorption layer of five different surfactants, namely, pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulfate, has been analyzed on the basis of molecular dynamics simulation results at two surface densities, namely, 1 and 4 μmol/m(2). The analyses have primarily focused on the question of how deeply, in terms of atomistic layers, the different surfactant molecules are immersed into the aqueous phase. The orientation and conformation of the surfactant molecules have also been analyzed. The obtained results reveal a clear difference between the immersion behavior of the alcoholic and ionic surfactants. Thus, alcoholic surfactants are found to be located right at the water surface, their apolar tails not being considerably immersed into the aqueous phase and the alcoholic headgroups being preferentially located in the surface layer of water. Ionic surfactants are immersed several layers deep into the aqueous phase, with headgroup atoms reaching the sixth-eighth and tail carbon atoms reaching the third-fourth subsurface layer in several cases. The observed difference is related, on the one hand, to the ability of the alcoholic surfactants of substituting surface water molecules in their lateral hydrogen bonding network at the water surface and that of their apolar tails for replacing dangling hydrogens and, on the other hand, to the energetic gain of the ionic headgroups if they are fully hydrated rather than being in contact with hydrocarbon tail groups.

Effect of hydrocarbon tail-groups of transition metal alkoxide based amphiphilic catalysts on transesterification

In liquid/liquid/solid (L/L/S) systems pertinent to two immiscible reactant liquids mixed with a solid catalyst, the reaction efficacy depends on the mass transfer limitations at the L/L/S phase boundary. Formation of an emulsion in such a system will likely reduce the mass transfer barrier significantly. The stability of such an emulsion system depends on the hydrophilicity of the head group of the catalytic emulsifier toward the more polar liquid reactant and the hydrophobicity of the tail group toward the more nonpolar liquid reactant. This study looks at the effect of the alkyl groups with varying carbon numbers in titanium alkoxide as a catalyst that also has emulsification (amphiphilic) properties to transesterify triglycerides in alcohols. All forms of oligomeric titanium alkoxides tested were highly basic. Those with smaller alkoxide groups (lower carbon numbers) tended to be more basic than those with higher carbon numbers. The chirality did not affect the degree of basicity of the alkoxides. The maximum ester yield noticed was 64.25% (with 63.85% selectivity towards transesterification) with titanium methoxide after 3 hours of reaction. It was observed that higher the number of carbon atoms in the tail group the lower the catalytic ability of the amphiphile towards transesterification. It is expected that longer the carbon-chain in the tail group stronger the emulsification ability of the amphiphile in oil-in-alcohol systems. However, when looking at the efficacy of the amphiphile for the combined emulsification and catalytic ability, it is apparent that the length of the alkoxide group needs to be compromised.

Simulations of Micellization of Sodium Hexyl Sulfate

Micellization of the ionic surfactant sodium hexyl sulfate has been studied using atomistic explicit-solvent molecular dynamics simulations with and without excess NaCl or CaCl(2). Simulations were performed at surfactant loadings near the critical micellization concentration. Equilibrium micelle size distributions and estimates of the critical micellization concentration obtained from the simulations are in agreement with experimental data. In comparison to the sodium dodecyl sulfate surfactant, the shorter alkyl chain of sodium hexyl sulfate results in increased disorder of the micellar core and water exposure of the hydrocarbon tail groups. However, water and ions do not penetrate into the micellar core even for these weakly micellizing surfactants. Excess NaCl is observed to have a minor influence on the micelle structure but excess CaCl(2) induces drastic changes both in the structure and the dynamics of the micellar system. Furthermore, in the absence of excess salt, sodium hexyl sulfate forms predominantly spherical, disorganized aggregates but an increase in ionic strength drives an increase in aggregate size and leads to prolate aggregates.

Fluorocarbon–hydrocarbon gemini surfactant mixtures in aqueous solution

The aqueous solution properties of oppositely charged gemini–gemini surfactant mixtures have been studied with a combination of static surface tension, dynamic light scattering (DLS) and cryogenic transmission electron microscope (cryo-TEM) measurements. Two gemini surfactants were employed in this study: one is a partially fluorinated cationic gemini surfactant, 1,2-bis[dimethyl-(3-perfluorohexyl-2-hydroxypropyl)ammonium]ethane dibromide, and the other is a novel anionic gemini surfactant having hydrocarbon tail groups ( N, N′-didodecyl- N, N′-dipropanesulfonylethylenediamine). Synergistic surfactant mixtures are frequently exploited to lower the mixed critical micelle concentration (cmc), however, such a decrease in the cmc is not observed in the current study. This suggests that limited mutual miscibility between the fluorocarbon and hydrocarbon tail groups plays a key role for the formation of the adsorbed layer, rather than attractive interactions such as electrostatic forces. On the other hand, spontaneous formation of vesicle-like molecular aggregates is suggested to occur as a result of their synergistic interaction in bulk solution.

Competitive sorption of anionic and cationic dyes onto cetylpyridinium-modified montmorillonite

Single-and multi-solute competitive sorptions of anionic dyes; Eriochrome Black T (EBT), Orange II (OR) and Methyl Orange (MO) and cationic dyes; Thioflavin T (TT), Methylene Blue (MB) and Crystal Violet (CV) onto montmorillonite modified with a cationic surfactant, cetylpyridinium chloride (CP), were investigated. In single-solute sorption, the sorption affinity, as represented by Freundlich sorption coefficient (K F) and Langmuir sorption capacity (q mL ), was in the order of EBT > OR > MO for anionic dyes and in the order of TT > MB > CV for cationic dyes. The sorption affinity of the cationic dye was higher than that of the anionic dye mainly due to the difference in sorption mechanisms: ion exchange to the bare montmorillonite surface plus two dimensional surface adsorption onto the pseudo-organic medium formed by the conglomeration of the long-chain hydrocarbon tail groups of the CP cation on the montmorillonite for cationic dyes vs. two dimensional surface adsorption only for anionic dyes. Three-parameter models (dual-mode and Song models) fitted better than the two-parameter models (Freundlich, Langmuir and Dubinin-Radushkevich models) due to the number of parameters involved. The conventional Dubinin-Radushkevich (D-R) model often used to classify sorption mechanisms based on the mean free energy were not able to explain the higher sorption of cationic dyes than anionic dyes. Among the tested models, the Song model was the best in predicting single-solute sorption in terms of the coefficient of determination (R2) and the sum of squared errors (SSE) values. Although both dual-mode and Song models fitted well to the sorption data, the results of asymptotic behavior analyses showed that Song model was better than dual-mode model in predicting sorption behaviors and in explaining sorption mechanisms. Competition between the solutes in the bisolute and trisolute system reduced the sorbed amount of each solute compared with that in the single-solute system. Generally the ideal adsorbed solution theory (IAST) coupled to the single-solute sorption model predicted the bisolute and tri-solute competitive sorption data favorably except a few bisolute systems.

Mixed Monolayer and Micelle Formation of Cationic and Zwitterionic Surfactant of Identical Hydrocarbon Tail in an Aqueous Medium: Interfacial Tension, Fluorescence Probe, Dynamic Light Scattering, and Viscosity Studies

We have studied the properties of the mixed monolayers and the mixed micelles formed from the mixture of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) and the zwitterionic surfactant dimethyldodecylammoniopropanesulfonate (DPS), that have the identical hydrocarbon tail but different polar head groups, by measuring interfacial tension, fluorescence, dynamic light scattering (DLS), and viscosity. From the results of interfacial tension measurements, the various interfacial and bulk parameters such as the maximum surface excess, the surface pressure at the critical micellar concentration, and the standard free energies of interfacial adsorption and of micellization have been evaluated. The deviation from the ideal of the mixed micelles has been discussed on the basis of Clint theory. The interaction parameters in the mixed micelles and in the mixed monolayers, and also the compositions of the mixed micelles and the mixed monolayers were obtained with the help of regular solution theory. From a static fluorescence quenching method, the mean micellar aggregation number of pure and mixed micelles was obtained. Microviscosity in the pure and mixed micelles was monitored by fluorescence polarization measurements using Rhodamine B (RB) as a fluorescence probe. The hydrodynamic radii of the micelles were obtained in the mixtures at different mole fractions of surfactant from DLS measurements. The change of viscosity with α of DTAB revealed a minimum, and it was shifted to a higher αvalue in the mixture of high concentration of surfactant. This minima suggested a structural change in the mixed micelles.

Micellar solubilization of ibuprofen: influence of surfactant head groups on the extent of solubilization

An important property of micelles with particular significance in pharmacy is their ability to increase the solubility of poorly soluble drugs in water, thus increasing their bioavailability. In this work, the solubilization of ibuprofen (IBU) was studied in micellar solutions of three surfactants possessing the same hydrocarbon tail but different hydrophilic head groups, namely sodium dodecyl sulphate (SDS), dodecyltrimethylammonium bromide (DTAB), and n-dodecyl octa(ethylene oxide) (C12EO8). The results showed that, irrespective of the surfactant type, the solubility of IBU increased linearly with increasing surfactant concentration, as a consequence of the association between the drug and the micelles. The 80 mM DTAB and the 80 mM C12EO8 micellar solutions resulted in a 16-fold increase in solubility of IBU when compared to the buffer solution, whereas the 80 mM SDS micellar solution resulted in a 5.5-fold increase in IBU solubility. The highest value of molar solubilization capacity (chi) was obtained with DTAB, chi = 0.97, followed by C12EO8 ,chi = 0.72, and finally SDS, chi = 0.23. However, due to the stronger tendency of the nonionic surfactant in forming micelles in solution, at the same surfactant concentration, we obtained the same solubility of IBU in both DTAB and C12EO8.

Modelling the dynamics of a minimal protocell container

This paper is a discussion on how reaction kinetics and three-dimensional (3D) lattice simulations can be used to elucidate the dynamical properties of micelles as a possible minimal protocell container. We start with a general discussion on the role of molecular self-assembly in prebiotic and contemporary biological systems. A simple reaction kinetic model of a micellation process of amphiphilic molecules in water is then presented and solved analytically. Amphiphilic molecules are polymers with hydrophobic (water-fearing), e.g. hydrocarbon tail groups, and hydrophilic (water-loving) head groups, e.g. fatty acids. By making a few simplifying assumptions an analytical expression for the size distribution of the resulting micelles can be derived. The main part of the paper presents and discusses a lattice gas technique for a more detailed 3D simulation of molecular self-assembly of amphiphilic polymers in aqueous environments. Water molecules, hydrocarbon tail groups and hydrophilic head groups are explicitly represented on a three-dimensional discrete lattice. Molecules move on the lattice proportional to their continuous momentum. Collision rules preserve momentum and kinetic energy. Potential energy from molecular interactions are also included explicitly. The non-trivial thermodynamics of large-scale and long-time dynamics are studied. In this paper we specifically demonstrate how, from a random initial distribution, micelles are formed and grow until they destabilize and can divide. Eventually a steady state of growing and dividing micelles is formed. Towards the end of the paper we discuss the relevance of the presented results to the design of a minimal artificial protocell.